1. Field of the Invention
The present invention relates to a polymeric composite separation membrane and its preparation method. The separation membrane of the present invention is produced by blending an amorphous thermoplastic resin, a semi-crystalline polymer and a compatibilizer which controls the dispersion state of the semi-crystalline polymer in the thermoplastic resin and adhesion at the interface. Then, a thin film of the blended mixture is prepared which is capable of increasing a selective permeation rate in such a manner that the dispersed semi-crystalline polymer is morphologically altered to serve as an obstacle to permeation, by which the permeation distance of chemicals (liquid or gases) is extended. When applied to a gas separation process, this enhances the selective permeability over the so-called trade-off limit. More particularly, a semi-crystalline polymer, which is hardly permeable to a gas or chemicals, is dispersed in an amorphous thermoplastic resin, and at this time, an appropriate amount of compatibilizer is added so that the semi-crystalline polymer has a uniform size and is in a well-dispersed state. Then, the mixture obtained from an extruder is passed through a die as a thin film, whereby, the semi-crystalline polymer phase is evenly dispersed and morphologically controlled in the film, and works as an obstacle to diffusion of chemicals or gases, to thereby morphologically altering path length of permeants. The chemicals or gases are separated according to interaction difference between the compatibilizer and permeant molecules, which induces diffusion time difference during which they pass the deformed path by the diffusion constant difference depending on the size of the chemicals or gas molecule and the interaction with the compatibilizer, thereby exhibiting a highly superior selectivity.
2. Description of the Related Art
Polymeric separation membranes are used for various separation processes which are technologically important, such as liquid separation, drug delivery, drug release, artificial kidney and lung, separation of air, separation of carbon dioxide from a natural gas, or separation of nitrogen and oxygen, or separation of hydrocarbon and hydrogen in a petrochemical process.
In the gas separation process, the basic factors determining the separation performance of polymeric gas separation membranes for a pair of gases, i.e., oxygen/nitrogen, carbon dioxide/methane or hydrogen/nitrogen, etc., are permeability constant and selectivity. Permeability constant is typically obtained by dividing the multiplied value of a gas concentration difference and the thickness of separation membrane by a pressure difference on both sides of the separation membrane. Selectivity is a ratio of permeability of two gases. That is, when the selectivity ratio is represented by A/B, ‘A’ is the permeability of the high permeable gas and ‘B’ is the permeability of the low permeable gas.
High performance separation membranes are desired to have a high permeability and a high selectivity, because a high permeability allows reduction of the area of separation membranes required for separating a certain amount of gas or chemicals and a high selectivity allows enhanced purity of product.
However, generally, if a separation membrane has a high permeability, it has a low selectivity, whereas if a separation membrane has a high selectivity, it has a low permeability.
According to Robeson's observation, as to separation of several pairs of gases, most polymer separation membranes have a negative slope in the correlation between the selectivity and the permeability (see Journal of Membrane Science, Vol. 62, 165, 1991, USA). That means the selectivity is reduced as the permeability is increased.
In addition, upon observation of performance of numerous polymer separation membranes, it has been revealed that most of the polymer separation membranes do not exhibit a performance higher than a certain level. That is, for a given permeability there exists an upper limit of selectivity, along which an inversely proportional relationship exists between the selectivity and the permeability all the time. The reason why such an upper limit exists is a natural result as the hard chains of a glass phase polymer substance screen the gas molecules. The slope of the upper limit has no relation to the chemical structure of the polymer separation membrane.
For the past 30 years, in order to simultaneously increase the selectivity and the permeability, most researchers in the separation membrane industrial field have directed their energy to synthesizing novel polymers (for example, see U.S. Pat. No. 5,725,633). However, though much research have been conducted, there has been no report of polymeric gas-separation membranes having a performance exceeding the existing upper limit. Very recently, we could make a polymeric-composite gas-separation membrane that has a selectivity going over the upper limit (see U.S. Pat. No. 6,517,606B2). In that case, the permeability of the prepared composite membrane was found to be rather low for practical use for the gas separation process.
The present invention is directed to a fabrication process for a polymeric, gas or chemicals, separation membrane that can overcome such an upper limit on the basis of a novel concept of a polymer composite instead of synthesizing a new polymeric material. The prepared membrane has a permeability that is high enough for application in real processes.
Therefore, an object of the present invention is to provide a process for fabricating a polymer blend allowing a uniform dispersion and having an improved interface adhesion by adding a semi-crystalline polymer to an amorphous thermoplastic resin and at the same time, adding a compatibilizer so that the compatibilizer works at the interface of the semi-crystalline polymer and the thermoplastic resin to lower the interfacial tension, to provide a process for fabricating membranes with a desired thickness by performing extrusion and drawing, and to produce polymeric gas- or chemical-separation membranes exhibiting a high selectivity in such a manner that an almost impermeable semi-crystalline polymer is uniformly dispersed in the film and is formed in a thin and long plate type, working as an obstacle to chemical or gas permeation to change the curvature of diffusing molecules so that they pass through the channel at the interface where the compatibilizer is located and the mixed molecules are separated because of the interaction difference with the compatibilizer.
Another object of the present invention is to optimize dispersion of a semi-crystalline polymer by using an optimum amount of the compatibilizer.
Still another object of the present invention is to utilize a drawing process, for fabricating thin film membranes to let the dispersed phase have a different axial ratio.
Yet another object of the present invention is to provide a method for fabricating a semi-crystalline polymer blended high-selective gas separation membrane.
Still yet another object of the present invention is to provide a method for separating a gas or chemicals in the liquid state by using the semi-crystalline polymer blended high selective separation membrane. For the liquid separation process, the liquids do not dissolve the membrane or its components.